ESTRO 2022 - Abstract Book

S560

Abstract book

ESTRO 2022

Conclusion HA hydrogel used as a tumour bed marker reduces inter- and intraobserver variability of BT boost target volume delineations. The study has proven, that the delineation variability results in D90 and V100 parameters falling below the recommended threshold. However marking the tumour bed with HA hydrogel minimized the reduction of impact on those parameters comparing to the control group.

OC-0630 Simulation of interstitial multi-catheter breast brachytherapy using a 3D surface imaging system

F. GASSA 1 , A. SERRE 2 , P. Pommier 3 , A. Seneclauze 3 , S. Mancini 2

1 Centre Leon BERARD, radiotherapy, LYON, France; 2 Centre Leon BERARD, radiotherapy, Lyon, France; 3 Centre Leon BERARD, Radiotherapy, Lyon, France Purpose or Objective Brachytherapy for breast cancer involves positioning several catheters through the skin into the breast tissues around the lumpectomy site. In order to get an acceptable dose distribution, the planning target volume (PTV) has to be geometrically covered by the catheters implanted. To plan the correct positions and reduce uncertainty between virtual and final implants, we developed a virtual simulation using Surface Guided Radiotherapy (SGRT) Technology. We propose to focus on the technical aspects of this new method. Materials and Methods On the pre-implant CT we delineated the PTV according to GEC ESTRO recommendations. A 3D virtual implant simulation of the catheters positions was performed using Monaco treatment planning system (Elekta AB, Stockholm) (TPS). For each virtual catheter, we created an external beam to obtain the isocenter. Markers were placed on all entry and exit points. We exported all the informations to a 3D surface patient setup system for alignment AlignRT ® (VisionRT, London) and torecord and verify the system (Mosaiq). In a radiotherapy treatment room equipped with AlignRT, we identified and marked on the skin all entry and exit points using the light field simulator of the linac. AlignRT assessed real-time patient positioning by comparison to a reference surface (patient external contour of the CT scan).The use of surface imaging could improve the reproducibility and accuracy of patient positioning with pre scan and reduce uncertainty in catheter simulation. At the end of this operation, Radiopaque skin markers were thereafter positioned on the breast surface on the entry and exit points. A CBCT was performed and exported to the TPS. We performed a registration between the CBCT and the pre implant CT using surgical scar and clips. The position of the markers was compared to entrance and exit points. Implantation was then carried out under local anaesthetic using skin marks of the catheter inlets and outlets. Final dosimetry was performed on post-implantation CT scan. Results For 4 patients, we compared coordinates form the entry and exit points defined on the pre scan and the CBCT (figure1) to evaluate the accuracy of the simulation. Mean differences observed were 0.2±0.3cm, 0.2±0.2cm and 0.3±0.4cm in left- right, supero-infero and antero-posterior (AP), respectively. A maximum difference of 1.1cm was obtained in the AP direction for external entry points. The obliqueness of the patient in this area amplified the shift